An article in a vacuum or low pressure environment is generally thermally isolated from its surroundings. Even if placed on or clamped to a support surface, only a limited number of microscopic points of actual thermal contact exist between the article and the support surface. In order to improve both the effectiveness and uniformity of thermal contact, it is known technique to introduce a thin layer of heat conveying gas between the article and the support surface.
This technique is frequently used in the deposition by sputtering or etching of thin films on silicon, ceramic or glass substrates. In these processes, it is sometimes desired to maintain a substrate at a low temperature in spite of heat generated by the sputter deposition or etching processes. In such cases, the thin layer of the heat conveying gas is used to transfer heat from the substrate to the support platform. The support platform is then cooled with suitable cooling means.
Alternatively, in sputtering processes, it is sometimes desired to maintain a substrate at a temperature higher than it would normally reach from the heat generated by the sputter deposition alone. In this case, the heat conveying gas film transfers heat from a support platform containing heating means to the substrate.
The same support platform may contain both cooling and heating means so that heat may be transferred either from or to a substrate as required.
The technique of thermally controlling a substrate by means of a heat conveying gas layer is disclosed in the following U.S. Pat. Nos.: 4,261,762, 4,457,359; 4,512,391; 4,542,298; 4,680,061; 4,743,570; 4,909,314 and 4,949,783.
A typical embodiment of this art is shown in FIG. 1. which is a section view through substrate and support platform and in FIG. 2 which is a plan view of the support platform surface.
A substrate 1 is placed on the engaging surface 2 of a support platform 3 which may contain cooling means 4 and heating means 5. Flange 6 and seal 7 connect the apparatus to processing chamber wall 8, providing for a vacuum or low pressure environment 9 around substrate 1 by sealing out an atmospheric environment 10 on the opposite side of plate 3.
A heat conveying gas, such as argon or helium, is introduced through port 11 and is distributed by channel 12. Port 13 is used to measure the pressure of the heat conveying gas, which may be adjusted to approximately 10 Torr in order to provide effective thermal transfer between substrate 1 and support platform 3.
Holding means 14, which may be a plurality of individual fingers or may be a continuous ring, retain substrate 1 against the force of the heat conveying gas pressure and maintain a close fit between substrate 1 and engaging surface 2. Because some heat conveying gas may escape in direction 15 at the perimeter 16 of substrate 1, thereby reducing gas pressure and thermal transfer, it is desirable that the gap between substrate 1 and engaging surface 2 is 0.003 or less.
In embodiments where heating means 5 are not used, a soft rubber seal may be placed near perimeter 16 to further reduce gas loss, but such seals are generally not used at elevated temperatures due to degradation of the seal material.
In order to maximize the area of thermal transfer between substrate 1 and support platform 3, the outermost portion 17 of gas distribution channel 12 is generally configured to conform closely to perimeter 16 of substrate 1, thus limiting the use of a particular support platform to substrates of a specific shape and size for which the support platform was constructed.